1. Field of the Invention
The present invention relates to a metal gasket, a vacuum flange for the metal gasket, and a vacuum seal structure using the metal gasket utilized in a vacuum seal at connections for piping and the like in a vacuum device.
2. Description of the Prior Art
Conventionally, a vacuum seal structure using a flange and a metal gasket is utilized with the object of removably connecting vacuum piping and the like used in various types of devices which operate under a vacuum.
The reason for using a metal gasket is that it is necessary to bake the piping and the like at a high temperature in order to quickly remove the gas which adheres to the inner walls of the vacuum vessels, vacuum piping, and the like, and to establish the processing conditions in the vacuum device.
Conventionally, a combination of a knife-edge-type metal-seal flange and a metal gasket made from oxygen-free copper is commonly known as a vacuum seal structure used with this objective. This seal structure is illustrated in
FIG. 8A and in FIG. 8B, and a perspective view of a metal gasket is illustrated in FIG. 9.
A conventional metal gasket 15 is shaped as a ring with a width w and a thickness t. From the requirements for using such a gasket, which will be later explained, the width w is normally formed quite large in relation to the thickness t. Specifically, as shown below in Table 1, a metal gasket with a width w of about 6 mm and a thickness t of about 2 mm, giving a t/w ratio of about 0.33, is used with a knife-edge-type metal-seal flange of an outer diameter of 70 mm. Also, a metal gasket with a width w of about 9 mm and a thickness t of about 2 mm, giving a t/w ratio of about 0.22, is used with a knife-edge-type metal-seal flange for which the outer diameter exceeds 70 mm.
TABLE 1 ______________________________________ Dimensions of gaskets used with conventional knife-edge-type metal-seal flanges Knife-edge-type metal-seal Gasket dimensions (mm) flange outer diameter Width (w) Thickness (t) t/w ______________________________________ 70 6 2 0.33 114, 152, 203, 253 9 2 0.22 ______________________________________
Two projecting sections 23a, 23a are formed on the respective inner peripheral sides of a pair of flange members 23, 23. Two pipes 21, 21 are welded in an airtight manner to the respective projecting sections 23a, 23a. Inner walls 29, 29 are formed on the opposing surfaces of the flange members 23, 23, provided with a difference in level so that the side on the inner periphery is lower. Two knife edges 25, 25 project into the inner walls 29, 29 toward the inner peripheral side.
When providing a seal between the two flange members 23, 23, the metal gasket 15 is accepted into the inner wall 29 of one of the flange member 23, the other flange member 23 is laid on top, and the two flange members 23, 23 are pressed together by means of a compression bolt 35 and a nut 37. The flange members 23, 23 which are compressed by the bolt 35 and the nut 37 transmits the compression force to the knife edges 25, 25 of the sealing section of the flange members 23, 23. Tip sections 25a, 25a of the knife edges 25, 25 bite into the metal gasket 15 so that the metal gasket 15 is subjected to plastic deformation. The outer surface of the metal gasket 15 is therefore pressed against the inner walls 29, 29 of the flange member 23, 23. As a result, an opposing force produced on the inside of the metal gasket 15 acts against the contact surfaces of the metal gasket 15 and the knife edge 25 and maintains the sealing performance.
Therefore, in this mechanism, for the seal performance to be reliable, the outer surface of the metal gasket 15 which has undergone plastic deformation must extend over the entire periphery of the metal gasket 15 and the contact with the inner wall 29 of the flange member 23 becomes, of necessity, an indispensable element. However, the movement of the metal gasket 15 in the radial direction is restricted because the knife edge 25 cuts into the gasket 15, and in order to make it possible to bring the entire periphery of the outer surface of the metal gasket 15 into contact the inner wall of the flange member 23, it is necessary that the outer surface of the metal gasket 15 and the inner wall 29 of the flange member 23 be already close together when the metal gasket 15 is being mounted on the flange member 23. For this reason the metal gasket 15 must be manufactured with a high degree of precision, but there are inherent limits to present fabricating methods using a pressing process. In addition, even if the outer surface of the metal gasket 15 and the inner wall 29 of the flange member 23 are almost touching over the entire periphery during temporary installation, in such a case the removal of the metal gasket after use is extremely difficult.
In such a case, as the result of the restriction of the movement of the metal gasket 15 in the radial direction because of being bit into by the knife edge 25 and various fabrication errors in the flange member 23 and the metal gasket 15, it is rare to have the outer surface of the plastically deformed metal gasket 15 contact the entire periphery of the inner wall 29 of the flange member 23. As a result, the implementation of a high temperature heating process (baking) of 100.degree. C. or greater for the vacuum device causes leaks to develop in the flange clamping section.
In addition, there is a strong possibility of producing a leak in a conventional knife-edge-type metal-seal flange, if the head of the knife edge is damaged. Therefore, the handling and the control of the flange is bothersome. Also, when the damage is extensive, reclamation is difficult.
Japanese laid-Open Patent Application 59-54857 discloses the provision of a notched section in the flange or a notched groove in the gasket itself for the relief of a deformed, swollen portion of the metal gasket when sealing is conducted by subjecting a metal gasket with a rectangular cross-section to compression deformation. However, this metal gasket is made from a soft material with the prerequisite that a large amount of swelling is produced from the deformation. The sealing principle is completely different from that of the present invention. Also, the metal gasket is deformed so that it adheres closely and completely covers over the sealing groove in the flange.